Apparatus for finding direction



April 29 1926. 1,581,622

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N. L. WALKER APPARATUS FOR FINDING DIRECTION Filed April 1? 1922 5 SheGtS-ShGGi 2 I L 1 Y/ MK 404729 V J PORT April .20 ,1'925.

, 1,581,622 W. L. WALKER APPARATUS FOR FINDING DIRECTION Fil Ap 12. 1922 5 Sheets-Sheet 5 I gwuahtoz Wz7ZiamZ.%Zker his afiomm s April 20 ,zrszs. 1,581,622 w. L. WALKER APPARATUS FOR FINDI NG DIRECTION Filed Apr il 12, 1922 5 Sheets-Sheet s C 1.2 4kg. 14

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351 his flitozum Patented Apr. 20, 1926.

unrrso STATES PATENT m me.

'WILLIAM L. \VALKER, OF NEW YORK, N. ASSIGNOR TO WALKER SIGNAL & EQUIP- MENT CORPORATION, NEW YORK, N. Y., A CORPORATION OF DELAWARE.

APPARATUS FOR FINDING DIRECTION;

- Application filed April 12, 1922. Serial No. 552,024.

To all whom it may concern:

lie it known that I, VILLLUI L. \VALKER,

a citizen of the-United States, residing in the borough of Manhattan, city, county, and State of New York, have inventedcertain new and useful ln'iprovements in Apparatus for Finding Direction, of which the following is a full, clear, concise, and exact description.

The object of my invention is to produce I an instrument or device and a method of operating the same whereby 'thevibratory energy received through the medium of microphones or similar receiving devices may be accurately compared as to intensity so that the direction and also the distance from which the waves emanate may be accurately determined and is in the natureof a compensator or balance. I

My invention is particularly applicable to the dctermimition of the direction of the source of the waves produced by submarine bells, tubes, cables and similar sources of .vihratory energy whereby the navigation of vessels is facilitated along the coast or at 25 other places in the ocean, or when entering harbors or other-more or less confined bodies of water. \Vith apparatus heretofore used for that purpose, difliculty has been experienced in determining the direction of a signal, particularly a sound signal, even though of comparatively high intensity, owing to the inefficient action of the receiving devices and the lack of means for accurately comparing the intensity ot'waves received at two or more separated points. The principal object. therefore, of my invention is to overcome the defects of apparatus heretolore used and to enable the navigator of a vessel to easily and accurately pilot the. vess'el on a definite course without the unnecessary alteration of the course.

In apparatus.heretofore used it has been necessary in attempting to navigate a vessel in a direction having a certain relation to a sound produced by a submarine hell or similar device, to swing the ship so that the in.-

tensity of the sound received on one side of the vessel may be compared with the intensity of the sound received on the other side bysnccessive observations. This means that the accuracy of the course is'determined by the sensitiveness of the listeners ear alone and has not been entirely satisfactory.

Briefly my invention comprises an apparatus whereby the pulsating or alternating currents produced by microphones and similar devices located on opposite sides of a vessel may be compared tor intensity to give an aural andvisual indication of the source of sound, its direction and distance through the medium of a single instrument or apparatus which may or may not be combined with a steering compass and which may be located in the pilot house, thereby enabling the pilot or navigator to receive the sound signals sent out by the submarine bell or similar device, and thereby ascertain its direction and steer an accurate course. To accomplish this result it maybe stated in a general Way that the respective microphones or similar receiving devices on the opposite sides of the vessel are connected preferably with a pair of coils provided with cores of magnetic material within which alternating or pulsating magnetic fields are produced in accordance with the currents generated by the sound wave acting upon the micro phones. In the present form of the apparatus I prefer to make use of the fact that the human car can detect and compare sounds just within the limit, of audibility much more easily and accurately than when the sounds are loudly audible. The respective fields of said field coils are therefore caused to pass respectively through one or preferably a pair of coils which are in turn connected with the indicating or receiving devices, such as the -usual head telephone used by telephone operators or radio operators. The alternating or pulsating'currcnts in one or both of th second pair of receiving coils may therefore be compared and such comparison will indicate the relative intensities in the magnetic field coils connected with the microphones, andso placed as to determine the direction "from which the sound is received. By providing the receiving or secondary coil or coils with a pointer or pointers, the direction from which the sound comes may be indicated on a scale or compass card. For a detailed description of receivers and various forms of'my invention.reference may be had to the following specification and to the accompanying a part thereof in which:

Fig. 1 is a plan view of a simple form of my invention illustrating the essential parts thereof. v

Fig. 2 is a sectional view taken substantially upon the line A-A, Fig. 1.

Fig. 3 is a vertical sectional view preferred form of my invention as at present devised. i

Fig. at is.a plan view thereof.

Figs. 5, 6, 7 and 8 illustrate schematically various connections between the telephone the receiving coils on the coinpensati ng, indicator.

Fig. 9 is a. schematic diagram showing drawings forming 'the switch members and contacts developed in one plane. and by which the various con- V duced between i such a switch,

"trating the in ,use of my invention.

inreceiving device nections illustrated in Figs. *to 8 inclusive may be accomplished by one switch.

Fig. 10 is an end .view of the casing of and Fig. 11 is an interior view of the switch, showing the end of the casingremoved. 1,,

Fig. 12 is a side elevation'of my improved compensating indicator applied to a. gyroscopic repeater compass and Fig. 13 is a plan view of the top of such a repeater compass showing parts broken away and indicating the interiormechanism schematically, and Figs. 1&',15, 16' and 17'-arediagrams illusethod of piloting a ship by the Referring to Fig. 1, the numerals 1 and 2 indicate the respective microphones or simisides' of the vessel or at similar places where it is desired to' observe or ascertain thedirection of sound or'other waves issuing from a distant point. Said microphones are connected with the usualenergizing batteries 3- and 4, and also with the coils 5 and 6 mounted on the annular magnetic ,core 7 The core 7 i's-preferably made of steel or iron, laminated to reduce eddy currents and is provided withthecentral pdl'e' or core portion. indicated at 8 which cooperates with theouter annular portion 7 to'produce magnetic lines of force between said parts, as indicated by the dotted lines of said'Fig. 1. The outer annular portion-7 is connected with the inner central pole or core 8 through a radial portion 9', which causes'the fields of.

force between the pole pieces toform complete magnetic circuits in the latter. shown in Fig. 1 the inner pole piece 8' being substantially concentric with the outer annular portion 7, the field of forceprothe said" parts 'is substantially uniform owing to the comparatively. small reluctance of the metalli Portion of the magnetic circuit; f I

' rows. of the s located at the opposite "the primary coils just audible in each telephone.

greater than that The coils 5 and 6 are preferably wound in opposite directions so'that the outer ends of the annular portions of "the core 7 will have the same polarity and the inner central portion. 8 .will have the opposite polarity, when the currents in said coils are flowing in the opposite directions through them. This will produce lines of force within the connecting piece 9 as indicated by the ar- Mounted so as, to be movable upon the annular core 7 are two coils 10 and 11 connected respectively with the telephone receivers 12 and 13. The coils 10 and 11 are also preferably wound in opposite directions with reference to each other and are preferably mounted on supporting arms 14 and 15, one of which is illustrated in section in Fig. 2, and are each provided with pointers 16 and 17 respectively which deterininetheir relative positions by reference to a scale 18 carried on the central pole piece orcore8. v

Referring now to Fig. 1,-it will be appreciated that, assuming the vessel is pointed directly toward the source of sound, the microphones land 2 will produce substantiallyequal pulsating currents in the coils 5 and (3.

These currentswill cause pulsating magnetic fields of substantially equal intensity to be produced in the two core which in turn \vill'induee substantially equal alternating currents in thereceiving -coils 10 and lland sounds of equal intensity in' the telephone receivers 12, 13. The coils 10 and 11 being near the upper ends of the annularcore 7 will be near if not at the limit of audibility of the sounds produced by the magnetization of the core, and if the sounds produced in the telephone are not audible, each of the coils is moved slightly toward until such sounds become Under the conditions first above pointed out, assuming that the'sounds are just audible in the positions of the coils indi'catedin Fig. 1, it is obvious that the apparatus willindieate that the vessel is pointed directly toward the source of sound or that the latter is dead halves of the annular,

ahead. Should the sound received by-one left hand side of "the annular core 7 will be side. Therefore, in order'to make the sounds in the telephone 13- 'ust audible to the listener, the coil 11 will have to be moved toward the coil 6 until such audibility is noted in the telephone 13. Supposing the coil 11 to be placed at'another point, such as that indicated by the letter B, Fig. 1. it is then apparent that the pointer 17 will indicate on the scale a point opposite the numeral 180, and the direction of the sound signal will, therefore, be at a point approximately that received by the micro- .phone 2, the pulsating field produced in theproduced in the right hand 7 midway between the latter numeral and 135, or about 157 degrees, thus indicating that the source of sound is approximately 22 or 23 degrees oil the starb ard bow of the vessel.

"hen this is determined, the navigator may then shift the course a ')proximately 22 or 23 degrees to starboard from the former course. and if the source of sound is then brought dead ahead, the coil 11 may be. moved to the position indicated in Fig. 1, and at which positions the sounds in the respective telephones will he just audible. If, l10\V6\'1;CUll 11 does not have to be moved to a position symmetrical with that of coil 10, then the angle between the course and the direction of the sound thereby found, and the vessel is then turned a few degrees to compensate therefor and another com parison made. This method of procedure indicates in general the manner of using my device and applies a so to modification hereinafter described. It is obvious that only one coil may 'be used to determine both points of minimum audibility by shifting the same successively from one such pointto the other but in such a case the pointer would be in alignment with the supporting arm of the coil rather than at an angle thereto as shown.

The different waysot connecting the telephones 12 and 13 with the coils 10 and 11 are indicated in Figs. 5 to 8 inclusive, Fig. 8 corresponding to the connections shown in Fig. 1. However, it may be desirable to connect the receiving coils 10 and 11 in series with each other and in series with each telephone as indicated in Fig. 5, in which case the coils 10 and 11 are preferably superposed one above the other or may be fixed 180 degrees apart, and the connection provided with a single pointer. This, however, requires comparison of the sounds in the receiver at two different points on each of the annular arms of the core as will be hereinafter described in connection with a modilication illustrated diagrammatically in Fig. 9. It also may be desirable to compare audibility of the sounds produced intlre respective ll'llCl'OPllOIlQS by comparing the intensity of the currents in the coils 10 and 11 sepa' rately. in which case the t'elephonesmay be shifted from one coil to the other, and the sounds compared successively instead of simultaneously.

Figs. 9, 10 and 11 illustrate one form of switch for, accomplishing the connections last above referred to by the simple turning of a knob.

Referring now to Fig. 9, the preferred form of the apparatus is illustrated sche matically at the upper right hand portion of this figure. The cores in this form of apparatus consist of two pairs of concentric laminated rods or bars of steel or iron superposed one above the other, as will be appreciated from a 'considerationof Fig. 3. One

pair of concentric bars, as indicated by the numerals 19 and 19 extend from the con necting piece 29 to their opposite ends 21 and 21, while the other or lower pair of bars 22 and 22 extend from the connecting piece 2; to their opposite ends 2 1 and 21. The coil R which correspomls to the coil (5 of Fig. 1 is fixed on the lower outer annular bar adjacent the connecting portion 21- whi e the coil R which corresponds to the coil 5 inFig. 1 is located on the upper outer annular member 19 adjacent the c lI1CL'tlIlg piece 20. The movable coils C and U correspond re peetively to the movable coils 11 and 10 of Fig. 1, the former being movable on the lower annular bar 22, while the coil C is movably located on the upper annular bar 19. V

It will be noted, both in Fig. 1 and in Fig. 9. that the inner and outer portions of the cores are concentric with each other,

but I do not wish to be limited to this atrangen'ient, as they. may be arranged so that different portions will be at varying distances from their respective branches between which the lines of force pass, thus varying the intensity of the fields'of force at different points, should it be necessary to change the spacing of the divisions on the scales or for other reasons.

referring new to Figs. 3 and 4 in which the n'icchanictil details of the device, as shown schematically in Fig. 9, are illustrated, the numerals 19 to 23 inclusive illustrate corresponding parts as in Fig. 9, and

,the coils C C R and R. correspond to those in Fig. 9. In Fig. 3 it will be seen that the supporting parts consists of a base ring 25 having projections 26 through which screws may be passed in order to secure the base ring to a suitable standard or to support the device as hereinafter to be referred to. The base ring 25 carries at one point, as indicated on the left hand side of Fig. 3, a raised portion 27 upon which the connecting piece 23 for the bars 22 .and 22' rest and to which it is secured by means of the screws 28 which engage a separator 29, upon the top of whichthe connecting piece 20 for the bars 19 and 19 rest. The connccting piece 20 issecurcd in position by a cross-bar 30 and screws 31 which also engage the separator 29. Similar separators, cross-bars and screws as indicated by the numerals 32, 33, 34 and 35 support and secure the bars 19 and 19, 23 and 23 at the other side of the coils R and R, as indicated in Fig. 4. The raised portions 27 connected with the base ring 25 carry at their inner ends supporting arms 86 which hold in position a bearing member 37. The separator- 29 carries at its inner side an arm 38 which supports an upper bearing member 39. The shaft 40, having nuts 41' and 12 on the opposite ends thereof, passes through the bearing members 37 and 39. A pressure spring 43 is interposed between the'nuts 42 and the bearing 37 in order to produce a slight pressure and friction on the bearf. gs 37 and 39 and the parts included between the same. I On the. shaft 40 is a flange or washer 40 which is preferably fixed thereto so as not to turn and this sep-aratcsthe hubs 44 and 45 carrying'arms.

46 and 47 which support-the coils'C and CT respectively. ,This support is'provided by tubes.

suitabletransverse plates 48 a nd 48 attached to the arm 46 and plates 49 and 49 attached to the "arm 47 and between which the respective coils are wound on suitable At the outer ends of the arms-'46 and 47 are transversepieces 50 and 51 re- 'spectively to which are attached two point- The upper'bar 51 carries.

ers 52 and 53. a screw threaded pin 54 having-a knurled knob 55. The lower endof the pin' 54 is adapted to enter a recess in bar 50 so that the two arms'46 and 47 and consequently the coils C and C may. be made to move inunison, being thereby rigidlyxconnecte'd.

The base ring also carries a series of brackets 56 which support an external rlng R and R and the coils C C, either when.

the latter are moved in unison or separately.

57 being divided into degrees or otherwise marked to enable the user to determine the relative angular position between the coils The te'rminalsof the coil R are connected with the binding posts. 58 and 58 while the coil R is connected with the binding posts 59 and 59.. The coil 0 is connected with binding, posts 60 and 60 on the tie plate 30', while, the coil C is connected with the binding posts 61. and 61 on. the same plate. These binding posts allow I the respective (oils to be connected with the respective switch contacts 61 of the switch as illustrated in Figs. 9, 10 and 11. The operation of this switch will now be described.

Referring to Figs. 9, 10'and 11, the numeral 61 indicates a series of strips of Spring metal, eight in number, adapted to engage the contact members 62 carriedon a rotary cylinder or drum 63, and in this case they are indicated as screw-heads in Fig. 11.

These contact -members are arranged on.

longitudinal lines or elements of the cylin-.

can) and (s) circuit arrangements indicated .in Figs. 5

der 63, as indicated by the numerals (5), which correspond'to the to 8 inclusivel The numeral 64 indicates the transverse connection between the left hand circumferential series .of contact members of Fig. 9. The dotted lines in Fig. 11indicate how the various connections shown in Fig. 9 are made through the interior of the cylinder 63.

Without tracing the various circuits, it

may be stated-that, when-the spring contact m mbers a e n e e wt th the ro ry course. for the change contact members 62 along the horizontal line (5) the'coils G C and the telephones P P will be connected as indicated in. Fig. 5. When said spring contact members 61 are in contact with the rotary contact members 62 along the line (6), Fig. 9, the 'coils C C and the telephones P will be arranged as indicated in Fig. 6.. In like .manncr the contacts along the line 7 will arrange the coils and telephones as lndicated in Fig. 7 and when contacts are made along the line (8). the coils and telephones will be arranged as in Fig. 8.

Referring now particularly to Figs. 3, 4v and 5, it willbe notedthat the coils C and C, in Figs. 3 and 4, are-superposed one above the other and rigidly connected by the screw pin 54.. Obviously this necessitates v the operation of the apparatus by a H1OV6 merit of the two coils at the same time. The coils C and C are preferably connected so as to opjposeeach other that is, when a current is generated in the-coil'G a current of opposite potential is generated in the coil C.

The operation of the apparatus as thus ar-' ranged is as follows: The observer first places the two rigidly connected armsand coils insuch a position that on one side of-the center line of the instru ment running forwardand aft of the ship. Assume that such a position is at the point S ordegrees on the starboard sideas in 'Fig. 4, the combined coils are then moved to the other side of the instrument to point where the sound is just audible at the port side, as indicated in dotted lines at the position P in Fig. 4. It will thus be seen that.

the two successive positions of the combined coils are separated by approximately 127 degrees. The mid position between the points the' sound in the telephones is just audible lu-S is therefore one half of thisposition plus. 90 or approximately 153 degrees on'the starboard side of the scale, as indicated by the time as the vessel proceeds, allowing of ress of the vessel.

If it lee-desired to locate the direction of the sound more accurately, the course of the vessel is changed, so that it will be about'26 of angle due to .prog

degrees to the starboard of the first course.

Successive readings are then taken from the scale, as above indicated, andif the angles between. the coils and the center line of the ship are equal, it will be known that the source of sound is dead ahead. On the other hand, if a, slight difference between the art'- gles of the coils with the center line of the ship still exists, the course is again changed until said angles are the same.

The last described method of determining direction by using the coils superposed and in opposition is more particularly applicable to instances where the source of sound is near at hand, say'within four or five miles, as the opposed connections of the coils tend 0 to reduce the effective action of the magnetic field in the concentric bars. In order to make the instrument more sensitive when the source of sound is at greater distances, say ten miles or more from the vessel or re ceiving station, the positions of the coils for the beginning of audibility may be determined by use of the switch of Figs. 9-ll, so that the operation between the coils and the telephones will be as in Figs. 6 and 7; -that is, the coil G is first connected with both telephones in series, and the audibility point determined. This is indicated by the position of the switch illustrated in Fig. 10. The switch is then turned so that the pointer 66 is at the position indicated by P, P, C, in Fig. 10, at which point the coil C will be in series with the telephones. The point of minimum audibility' is then determined for the coil C. The angle between the two positions of the respective coils thus found is then used to indicate the course, and the subsequent piloting of the vessel, as before described. The reason that the apparatus is more sensitive by using this latter procedure is that theresistance of one coil only is interposed between the telephones and their opposed currents eliminated. the same procedure is followed with thetelephones connected as in Fig. 8, in whichposition each coil is more sensitive to the magnetic field of force than in either of the prior cases, and in this connection it is obvious that the sound produced in the telephones by the coils C and C may be compared siinul taneously by a skillful operator. If the operator cannot compare the sound siinultane ously, it is only necessary to alternately connect and disconnect each telephone with its respective coil until the positions of the respective coils are determined with regard to the -minimum audibility in each coil, in which case the course is'deterinined as before. By this arrangement the resistance of the circuits in which each coil is placed is reducedby one half the resistance of the telephone circuit, since the telephones are independently connected with the coils and the sensitiveness of the apparatus is therefore still further increased.

, If it be desired that the navigator may observe the course of the ship accurately by compass, and at the same time observe the direction of the sound relative to the, course of the ship and also relative to the true 6 north, the compensating indicator may be Substantiallymounted upon the top or frame of a compass,'preferably a gyroscopic repeater compass of the type indicated in Fig. 13, which is controlled electrically in the manner well known inthe art, from a master gyroscopic compass located in another part of the vesupon a vertical-column or standard 73.-

Screws 7.4 passing. through the projections 26 referred to in connection With Fig. 3 retain the base ring 25 in position upon the repeater compass.

Referring now to Figssl l to 17 inclusive,

these figures illustrate the various steps in.

ascertaining the direction of the source of sound by the manipulation of the compensating indicator as previously described. In each of these instances, the coils C and C are indicated as being independently movable with the telephones connected as showniii Fig. 8. Fig. 14 illustrates the relative positions of the course, the bell and the ship when the first observations are taken in either one of the ways previously described. Fig. 15 illustrates the second observation where the course has been changed to more nearly approximate that of the source of sound, while Fig. 16 indicates the second change in course through the small remaining angle, so that the source of sound is dead ahead.

Fig. 17, .comparedwvith Fig. 16, illustrates, -roughly, how the distance between the source of sound and the vessel may be determined. Obviously, when the vessel is nearer the source of sound, the effect of the sound waves upon the microphone coils R and R will be greater, thus establishing stronger variations of. the magnetic fields in the annular cores. This, in turn, produces stronger induced currents in the coils C and C, and consequently louder sounds in the telephones. The points of minimum audibility will therefore be indicated by positions of the movable coils which are further removed from the fixed coils than when the distance of the source of sound is far ther away. Obviously, iiiterinediate positions will be indicated as the distance from the source of sound varies. Thus, after the initial use of the apparatus at varying known distances from sources of sound, the scale may be calibrated or may be used to indicateapproximately the distance of the vessel from, the source ofsound in subsequent navigational observations. Such calibration or readings should of course be so made as to take into account the various consel, such electrical control being schemati inc nections ofthe telephones which affect the' sen'sitiveness as indicated by the different connections in Figs. 5 to 8 lnclusive.

' It is obvious that instead of the receiving devices in the form of microphones, other receiving devices, such as loops or coils may be 'used for detecting radiant electric or magnetic energy with or without amplifyingdevices, as is already known in the art.

Having thus described the principles of operation and construction of my lnventron, I do not wish to be understood as being ilimitedto the details of form and arrangement of parts herein set forth, for VfillOllS changes may be made by those skilled in the art without departing from the spirit and tion and distance of sources of vibratory Scope of y invention as set forth in the following claims. 1

:Whfit I claim and desire to prot t b v ters Pat 1 1 1. Apparatus for determining the direcenergy comprising, electrical receiving devices located at s 'aced points and adapted to produce 'variab e electric currents in accordance-with the intensity of the received 7 waves, coils connected therewith for pro- I ,Hducingf magnetic Y fields whoselntensities varym proportion to said currents, spaced cores ofmagnetic material therein adapted to form air gaps having fields of progres- 'tionand distance of sources of vibratory 4o energycomprising, electrical receiving de- ;vices located at spaced points and adapted cause variation in electric currents in accordance with the intensity of I sity the received waves, coils connected therewith for producing magnetic fields whose ntensities vary in proportion to sa1d currents,

.spaced coresof magnetic material therein adapted to form air gaps havin fields of progressive intensity, coils mova lewithin said fields and adapted to be, placed at points the'reimwhere the intensities. thereof are equal, and means 'for determining thestrengths of the currents in-said coils and for comparing the latter at various points in'said field. 1

3. Apparatus for determining the direction and distance .of vibratory "energy comprising, electrical receiving devices located at spaced points and adaptedto vary electric currentsin accordance with the inten of the received waves, coils connected therewith 'forproducing magnetic fields whose intensities vary in proportion to said currents, spaced cores of magnetic material along said cores.

fields of progressive intensity, coils movable within said fields and adapted to be placed respectively .at separated points therein where the intensities are substantially the same, and meansfor audibly comparing the respective strengths of the currents in said movable coils.

-4. Apparatus accordin to claim 2- in-\ which said movable coi 5. Apparatus accordingto claim 3 in which said movable coils are adjustable along said cores.

6. Apparatus according to claim 1, in which the air gaps are of equal length between the cores so as to produce fields'of force in said cores of substantially'uniformly varying intensity.

s are adjustable 7. Apparatus according to claim 2, in Y which the air gaps are of equal length between the cores so as to produce fields of force in said cores of substantially uniformly varying intensity.

8. Apparatus according to claim 3, 'in which the air' gaps are of equal lengthbetween the cores so as to produce fields of force in said cores of substantially uniformly varying intensity;

9. Apparatus according to claim 1, in

Which'themagnetic fields are generated in said cores of magnetic material having poles equally spaced apart transversely, said movable coils-being adjustablealong said poles,

10. Apparatus according to claim 2 in which the magnetic fields are generated in said cores of magnetic material having poles equally spaced apart transversely, said movable coils being adjustable along said poles.

11, Apparatus according to claim 3 in which the magnetic fields are generated in said cores of magnetic material having poles equally spaced apart transversely, said movable coils being adjustable along said poles. 12. Apparatus according to claim 1, in

which the magnetic fields are generated in said cores of ma etic material having concentric poles, said movable coils being adjustable along said poles.

, having-poles annular 1n sha e and supen,

posed in parallel planes, sai "movable coils I being' adjustable along said poles. lZhf-BI'BIII adapted to form air gaps having 16. Apparatus "according to claim 2 in 13. Apparatus according to claim 2, in

which the respective magnetic fields are gen erated in said cores of magnetic material having poles annular in shape and superposed in parallel planes said movable coils being adjustable along said poles.

17. Apparatus according to claim 3 in which the respective magnetic fields are generated in said cores of magnetic material having poles annular in shape and super posed parallel planes, said movable coils beingfjadjustable along said poles.

18. Apparatus according 'to claim; -2 in which the means for comparing the strength of currents are provided with a. switch whereby said movable coils may be connectedin series with each other and'with said means,' or each alone in series with said means or each: in parallel respectivelyjwith portions of said means.

,19.'Apparatus according to claim 3 in which the means for comparing the strength" of currents are provided with a. switch le coils may be connected.

whereby said mova said telephones may with one of said telephones may be connected in series,

or" either of said coils alone may be connected in series with said telephones, or each of be connected with one of said coils simultaneously. i

21.. Apparatus accordingto claim -3 in which the means for comparing the strength of currents are a pair of head telephones,

anda switch whereby said movable coils and saidtelephone vmaybe connected in series,

or either'of said coils alone may be connected-inseries with said telephones,' or each of said telephones may be connected said coils simultaneously. WILLIAM L. WALKER. 

